The present invention relates to improved set retarders for foamed cements, foamed cement compositions containing the improved set retarders, and to improved methods of cementing in subterranean zones with set retarded foamed cement compositions.
Foamed hydraulic cement compositions are often utilized in cementing subterranean zones penetrated by well bores. For example, foamed cement compositions are used in primary well cementing operations whereby strings of pipe such as casing and liners are cemented in well bores. In performing primary cementing, a cement composition is pumped into the annular space between the walls of a well bore and the exterior surfaces of a pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented. Examples of foamed hydraulic cement compositions are described in U.S. Pat. Nos. 5,897,699 and 6,063,738.
The cement compositions utilized for cementing in subterranean zones or formations penetrated by well bores must often be lightweight to prevent excessive hydrostatic pressure from unintentionally fracturing the zones or formations. In addition to being lightweight, a foamed cement composition contains compressed gas which improves the ability of the cement composition to maintain pressure and prevent the flow of formation fluid into and through the cement composition during the transition time, i.e., the time during which the cement composition changes from a true fluid to a hard set mass. Foamed cement compositions are also advantageous because they have low fluid loss properties.
When cement compositions are utilized for cementing in deep hot subterranean zones, a set retarder must be included in the cement composition to increase the pumping time of the composition and prevent premature thickening or setting before placement in the zones to be cemented. Examples of set retarders which have heretofore been utilized in non-foamed cement compositions include, but are not limited to, lignosulfonates, sulfomethylated lignosulfonates, hydroxycarboxy acids, mixtures of sulfomethylated lignosulfonates and hydroxycarboxy acids, acrylic acid/2-acrylamide-2-methyl propane sulfonic acid copolymers and the like. While the foregoing set retarders function well in non-foamed cement compositions, they do not function well in foamed cement compositions because they have dispersing properties. That is, when used in a foamed cement composition, a set retarder having dispersing properties causes the cement slurry to be thin which in turn causes the foam to be unstable and either break or significantly decrease the viscosity of the foamed cement slurry which in turn prevents the desired foamed cement composition low density from being achieved. While carboxymethylhydroxyethylcellulose (CMHEC) has heretofore been used in foamed cement compositions as a set retarder, the high concentrations of CMHEC required causes the foamed compositions to have undesirable high surface viscosities.
Thus, there are needs for improved methods, set retarding additives, and set retarded foamed cement compositions for cementing subterranean zones penetrated by well bores.
The present invention provides improved set retarders for foamed cement systems, and cement compositions formed therewith. The set retarders comprise a blend of a sulfonated lignin functioning as a set retarder and an alkali lignin containing 0-3.5% organic sulfur functioning as a stabilizer. The sulfonated lignin may be either a lignosulfonate (sulfite lignin), or a sulfonated alkali lignin with an organic sulfur content of about 2% or greater. The sulfonated lignin may be used xe2x80x9cas-isxe2x80x9d or in a further modified form so long as the organic sulfur content is about 2% or greater, preferably 5% or greater and most preferably 2-12%. The stabilizing alkali lignin may be an unsulfonated kraft lignin or a kraft lignin with a low organic sulfur content, i.e., 3.5% or less. The set retarder and stabilizer ingredients may be blended in a ratio of about 6:4 to about 8:2, and blending may be accomplished by dry blending or by mixing solutions of the two components together and spray drying.
The present invention also provides improved methods for cementing in subterranean zones penetrated by well bores which meet the needs described above and overcome the deficiencies of the prior art. The improved methods of this invention are basically comprised of the following steps. A foamed cement composition is prepared comprised of hydraulic cement, a non-dispersing set retarder comprised of a mixture of a sulfonated lignin containing at least 2% organic sulfur and an alkali lignin containing 0-3.5% organic sulfur, sufficient water to form a slurry, sufficient gas to foam the slurry and a foaming and foam stabilizing surfactant mixture. The foamed cement composition is then placed into a subterranean zone, and the foamed cement composition is allowed to set into a solid mass therein.
It is, therefore, a general object of the present invention to provide improved methods of cementing in subterranean zones penetrated by well bores.
Another object is to provide improved set retarder additives for foamed cements, as well as to provide improved cement compositions incorporating such set retarders for use in the completion and remediation of subterranean wells.
A further object of the present invention is to provide improved methods of cementing in subterranean zones penetrated by well bores with a foamed cement slurry containing a non-dispersing set retarder.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.